<!DOCTYPE html>
<html>
<head lang="en">
    <meta name="viewport" content="width=device-width,initial-scale=1.0">
    <meta charset="UTF-8">
    <title></title>
    <style>
        .a1 {
            max-width: 1000px;
            margin: auto;
            /*box-sizing: border-box;*/
            background-color: indianred;
        }

        .a2 {
            max-width: 1000px;
            background-color: blueviolet;
            margin: 100px auto;
            /*box-sizing: border-box;*/
            padding: 200px;
            border-width: 100px;
        }

        .a3 {
            position: fixed;
            bottom: 0;
            right: 0;
            width: 100px;
            background-color: greenyellow;
        }
    </style>
</head>
<body>

<div class="a1">
    <p>abc</p>
</div>

<div class="a2">
    <p>abc</p>
</div>

<div class="a3">
    <p>aaa</p>
</div>
</body>
</html>